The present invention relates to a method and apparatus for calibrating a reaction injection molding machine to dispense each of a plurality of reactive components in proper proportion and thereby correctly mix a reactive molding material, all with minimum interrruption of normal production operation of the apparatus.
Reaction injection molding, also called liquid injection molding, is a technique for combining reactive liquid polymeric components and injecting them into a closed mold under pressure where they set to form a finished product. The component combination in, for example, a high density polyurethane foam formulation is achieved by directing streams of two or more reactive polymeric liquid components, each other high pressure, to cause their impingement at a common point in a mixing chamber of a mixing head. The resulting component impact creates a homogeneous mass of material in the mixing chamber which is then injected under pressure into a closed mold to which the mixing head is connected.
This reaction injection molding technique is distinguished from conventional plastic foam open molding technique in that a completely closed system is maintained for the components from withdrawal from their respective supply tanks to their injection in admixed condition into the closed molds. The technique permits formation of more intricate, delicate products, having narrow or thin sections that could not ordinarily be formed by conventional low pressure or open mold techniques; also it permits use of components of greater viscosity than is practical in the low pressure, open mold technique.
However, reaction injection molding systems have certain problems. Accurate metering of the reactive components is important to insure that these are combined in proper quantities, since slight variations in the polymer mix can result in defects in the finished product, causing rejection of such product. Moreover, the relatively high pressures used in reaction injection molding systems to obtain adequate mixing cause volume changes to occur in the system equipment during an injection cycle. These are due, for example, to component conduit or hose expansion and contraction arrising from pressure pulses at the start and termination of the injection cycle. This produces a condition known as "leading" or "lagging" of one component feed relative to another during the injection time interval, with resulting improper ratio of mixed components being introduced into the mold. When an aberation from the desired ratio of component mix in the finished molded product does occur, the physical properties of the finished product are adversely affected.
U.S. Pat. No. 3,551,974 (Jennings) discloses an apparatus for polyurethane molding that controls the rate at which the mold is filled. The apparatus includes individual flow meters that monitor the mass flow rate of each component as it is conducted to a mixing head. A signal proportional to the combined mass flow rate is generated by a circuit including tachometers connected to the flow meters, and is utilized to determine the rate at which polyurethane is deposited in the mold. The quantity of each component fed to the mixing head is not controlled to regulate the component mixture, however.
Still other prior art methods and apparatus attempt to measure the flow characteristics and properties of the reactive components at the mixing head. This approach however necessitates interrupting production of the products, since the mold must be disconnected from the head and the mixing head dismantled to gain access to the individual components for measurement of their respective flows. Furthermore, under such conditions, the flow will not be subject to the operating parameters of pressure, flow restrictions, etc., imposed during actual molding injection operations. Such approach therefore is not only expensive in terms of labor and time, but has inherent inaccuracies.
U.S. Pat. No. 3,608,001 (Kowalski et al) discloses a process for producing a polypropylene having a desired average molecular weight and molecular weight distribution in an extruder-reactor. The polypropylene is sampled to regulate the conditions in which its components react, without dismantling the mixing head, but only after combination of its components has already occurred. Again, therefore, no individual check of individual component feed rate is involved.
U.S. Pat. Nos. 3,499,387 (Zippel) and 3,788,337 (Breer) disclose other molding devices, but these also fail to disclose an apparatus or method of calibration wherein samples of individual component quantities are measured prior to mixing; and more especially there is no disclosure of sampling done under conditions simulating actual operating parameters.